Constant velocity universal joints are broadly classified into fixed types that only permit an angular displacement between an input shaft and an output shaft, and sliding types that permit both an angular displacement and an axial displacement. The model of the constant velocity universal joint is selected according to the purpose and conditions of use of the respective cases.
FIG. 5 shows an example of a fixed-type constant velocity universal joint (hereinafter, simply “joint 101”). Based on the joint 101 as an example, a conventional constant velocity universal joint will be described. The joint 101 mainly includes an outer member 106 with a plurality of raceways 105 formed an a spherical inner surface (inner diametral surface) 103 of a cup portion 104, an inner member 110 including an inner race 109 with a plurality of raceways 108 formed on a spherical outer surface (outer diametral surface) 107 and a shaft 102, a plurality of balls 111 disposed between the raceways 105, 108, and a retainer 113 with a plurality of ball pockets 112 circumferentially aligned at a predetermined interval for retaining each of the balls 111 (for example found in JP-A No. 2003-130082).
FIG. 6 shows the joints 101 applied to a steering apparatus 71 of a vehicle. The steering apparatus 71 includes the joints 101 between an input shaft 73 connected to a steering wheel 72 and a steering gear 74, so as to transmit a rotational torque exerted by the steering wheel 72 to the steering gear 74, with an operational angle between the input shaft 73 and the steering gear 74. The steering apparatus 71 may be an electrical power steering apparatus (EPS) that provides auxiliary power via a motor, or a hydraulic power steering apparatus that provides auxiliary power via a hydraulic pressure.
In FIG. 6, numerals 50a to 50c designate yokes serving as connecting members. The yoke 50a connects a shaft 115 of the outer member 106 extending from the joint 101A disposed closer to the steering wheel 72, and the shaft 102 of the inner member 110 extending from the joint 101B disposed closer to the steering gear 74, so as to transmit the torque.
The yoke 50b connects the input shaft 73 and the shaft 102 of the inner member 110 extending form the joint 101A closer to the steering wheel 72, so as to transmit the torque.
The yoke 50c connects the shaft 115 of the outer member 106 extending from the joint 101B closer to the steering gear 74 and the steering gear 74, so as to transmit the torque.
Such yokes 50a to 50c and other connecting members that connect the joint 101 and a shaft external to the joint are formed separately from the outer member 106, the inner member 110 and so on, to be attachable thereto so as to transmit the torque, from the viewpoint of convenience in processing and flexibility in changing design specifications.
Conventionally, as described above, the shaft 102 of the inner member 110 and the yoke 50 serving as the connecting member are separately manufactured and then coupled. More specifically, the shaft 102 is fitted to the yoke 50 via a torque transmission unit such as a spline so as to transmit the torque, and the shaft 102 and the yoke 50 are fixed to each other by press-fitting, welding, bonding or fastening with bolts.
Regarding the steering apparatus 71, it is necessary to address the issue of how to prevent a rotational backlash in order to precisely transmit the rotation of the steering wheel 72 to the steering gear 74. When applied to a use where rotational backlash is undesired, it is necessary to prevent rotational backlash between the inner member 110 and so on of the joint 101 and the yoke 50.
When the yoke 50 is press-fitted to the inner member 110 and so on, the mutual interface in the torque transmission unit serves as an interference fit, which restricts the rotational backlash between those members with only the torque transmission unit.
Increasing the interference, however, leads to degraded assembly work efficiency because an enormous pressing load is required for fitting the members. That is where the difficulty lies in controlling the mutual interference of the torque transmission unit to be within an appropriate range.
On the other hand, when the inner member 110 or the like and the yoke 50 are fixed by welding, although the rotational backlash does not emerge between the members, the welded portion is exposed to high temperature, which incurs distortion that may lead to degradation in precision or a crack inside the joint, thereby lowering the yield of the joint 101. In addition, when the inner member 110 and the yoke 50 are fixed by bonding, secular deterioration of the adhesive may incur an increase in a gap in a rotation direction between the members of the torque transmission unit, thus provoking the rotational backlash.